The German Patent 35 12 709 describes a generic method for the construction of wells in unstable rock, whereby between the filter gravel introduced in the area of the filter-pipe segment, referred to hereinafter simply as the "filter pipe", and the remaining portion of the annular space between the lengthening-pipe segment, hereinafter referred to simply as the "lengthening pipe", and the inner wall of the drill hole, a pumpable permanent-plastic or permanent-elastic seal is introduced. The annular space between the lengthening pipe and the inner wall of the drill hole is filled and sealed for sample taking.
With this known method a water sample can be taken only in the area of the actual filter pipe and the water bearing strata which may possibly exist above the filter pipe are hermetically and hydraulically sealed off from the openings in the filter pipe, due to the sealing of the annular space between the inner walling of the drill hole and the lengthening pipe.
The known method has the disadvantage that with this kind of well construction via hydraulic-system drilling, it is not possible to extract water in water-bearing rock strata with poor load-carrying capacity, since when the filter pipe is located in a water-bearing stratum which is loaded with noxious substances, it is necessary to redrill the entire drill hole. Before that, it is necessary to extract the filter pipes and the lengthening pipes. However, the extraction of lengthening pipes is almost impossible to perform, since due to the friction between the sealing material filling the annular space and the outside of the lengthening pipe, such a resistance against the extraction motion is created, that the drill holes are normally destroyed and collapse.
U.S. Pat. No. 4,366,862 describes equipment with which it is possible in sturdy mountain rock to define an underground area via spaced-apart packers, which can be pressed against the walling of the drill hole, in order to take out a sample of the liquid. Packers can be used only in sturdy rock. In rock that is not sturdy, where it is necessary to work with additives to the hydraulic drill fluid in order to stabilize the drill-hole wall, the packers destroy not only the stabilizing coating on the drill-hole wall, but also the drill-hole wall itself.